1. Field of Invention
This invention relates to infrared transparent glasses containing tellurium which have solubility for rare-earth ions.
2. Description of Prior Art
Infrared transmitting materials are known and comprise a variety of different materials including crystalline halides, silica and fluoride glasses, and chalcogenide glasses. Crystalline halides undergo plastic deformation and are hygroscopic, requiring cumbersome containment apparatus for IR systems applications. Laser glasses have been developed as host materials for rare earth ions but mainly for applications operating at wavelengths less than 3 microns. Silicate and fluoride glasses have been developed as optical fiber amplifiers but are limited by their high phonon energies relative to chalcogenide glasses. It is widely recognized that longer emission lifetimes and hence, higher efficiencies, are achieved with lower phonon energy host materials for rare earth ions. The class of chalcogenide glasses includes sulfides, selenides and tellurides, respectively, with increasing mass and weaker bonding strength. With increasing mass and lower bonding energy, the glasses transmit to longer wavelengths due to the lower phonon energies. Sulfide glasses are well known and Harbison et al. in U.S. Pat. No. 5,599,751, herein incorporated by reference, describe an infrared transmitting germanium sulfide glass that would tolerate the addition of rare earth ions in the glass.
Telluride glasses have been based upon As--Te and/or Ge--Te compositions. Glasses based upon As--Te and/or Ge--Te lack the ability to dissolve rare earth ions. Martin in U.S. Pat. No. 4,942,144 teaches that chalcogenide IR transmitting glasses can be made with the following formula: EQU MX +M'.sub.2 X.sub.3 +Si X.sub.2
where M represents a metal selected from calcium, strontium, barium, zinc and lead. M' is the metal used to form network bridging and represents aluminum or gallium and X represents S, Se or Te. A major problem with these glasses is that when X represents Te, the glass stability against crystallization is poor, making the drawing of optical fiber from these glasses very difficult.
Lucas et al in U.S. Pat. No. 5,352,639 teaches that tellurium and selenium halide-based glasses and optical fiber can be made with the following formula: EQU Te.sub.w Se.sub.y X.sub.x Z.sub.z (I)
where X represents at least one halogen selected from chlorine, bromine and iodine. Z represents at least one element selected from arsenic, antimony and bismuth. These glasses have sufficient stability against crystallization to permit drawing optical fiber, but they lack the ability to incorporate, or solubilize rare earth ions.
U.S. patent application entitled "Infrared Transparent Selenide Glasses" bearing Ser. No. 08/816,204 and filed Mar. 14, 1997, discloses a process for making selenide glasses and selenide glass compositions comprising at least 5 mol % alkaline earth selenide, 20-70 mol % germanium selenide, and 0.5-25 mol % Group IIIA selenide selected from gallium selenide, indium selenide and mixtures thereof. These glasses have demonstrated the solubility of rare earth ions, and are the longest wavelength IR transmitting chalcogenide glasses that would tolerate the addition of rare earth ions until the now disclosed telluride glasses.